In recent years, one of the important development trends of the switching power supplies is miniaturization. In a switching power supply, a magnetic component accounts for a large proportion in volume, weight, loss and cost, so design and optimization of the magnetic component is crucial. One effective means to reduce the volume of the magnetic component and to improve a power density is to increase a frequency of the switching power supply, which is a focus of the conventional magnetic design. In high-frequency magnetic design, the most widely-used winding is the PCB winding, because compared with a winding having a conventional winding structure, the PCB winding has huge advantages in terms of fabrication, cost, reproducibility and modularity. As far as a magnetic core is concerned, ferrite has a low loss and cost compared with other magnetic materials, so in the conventional design of transformers and inductors, ferrite is mostly used as the magnetic core. Since the ferrite has a high permeability, and in general, a relative permeability of the ferrite reaches up to hundreds or even thousands, in order to achieve a desired inductance of the magnetic component, an air gap may be cut in the magnetic core to bear the magnetic pressure drop and to store energy such that the magnetic component may not be saturated.
FIG. 1 is a sectional view of an EI magnetic core cut with an air gap seen from a direction parallel to a magnetic flux direction. In the drawing, dash lines represent a magnetic flux, arrows represent a magnetic flux direction which may vary with a current direction, and an air gap G is formed between an upper cover board and core columns 2. In a window W between the core columns 2 of the magnetic component, a magnetic field strength is relatively large in the vicinity of the air gap, and relatively small in the vicinity of the magnetic core. In addition, the magnetic flux may diffuse toward inside of the window W when the magnetic flux passes through the air gap, as shown by a magnetic induction line at point A in FIG. 1, which may affect the winding in the vicinity of the air gap G and increase a loss of the winding. Under a high-frequency eddy current effect, not only a skin effect and proximity effect loss of the winding coil may significantly increase, but also a large eddy current loss may be generated by the magnetic flux diffusing in the vicinity of the air gap.
In order to reduce the eddy current loss generated at the winding by the air gap, in the related art, leeds wire with a relative small diameter is adopted to form the winding for improving such condition. However, the winding formed of leeds wire has a low window filling rate, is time consuming in fabrication, and tends to be broken due to the small diameter.
In the related art, a magnetic core with a low permeability is adopted to avoid the air gap. However, the magnetic core with a low permeability, such as a magnetic powder core, has a loss that is far larger than that of the ferrite. Also, in the related art, a plurality of distributed air gaps may be cut on the ferrite magnetic core to reduce the effect of the air gap. However, this process is complex and time consuming. Moreover, the winding may be arranged at a position far away from the air gap, to make it far away from an area with a large magnetic field strength. However, this method obviously compromises the volume.